1. Field of the Invention
The present invention relates to compositions that reduce the reflection of exposure radiation from a substrate back to an overcoated resist layer. More particularly, the invention relates to an antihalation composition characterized in one aspect by containing a thermally activated crosslinking agent.
2. Background Art
Photoresists are used for transfer of an image to a substrate. A coating layer of a photoresist is formed on a substrate, and the resist layer is then selectively exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the resist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of the substrate.
A photoresist can be either positive-acting or negative-acting. For most negative photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the resist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble. The background of photoresists are described by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, ch. 2, 1975, and by Moreay, Semiconductor Lithoaraphy, Principles, Practices and Materials, Plenum Press, New York, ch. 2 and 4, both incorporated herein by reference for their teaching of photoresists and methods of making and using same.
A major use of photoresists is in semiconductor manufacture where an object is to convert a highly polished semiconductor slice, such as silicon or gallium arsenide, into a complex matrix of electron conducting paths, preferably of micron or submicron geometry, that perform circuit functions. Proper photoresist processing is a key to attaining this object. While there is a strong interdependency among the various photoresist processing steps, exposure is believed to be one of the most important steps in attaining high resolution photoresist images.
Reflection of the activating radiation used to expose a photoresist often poses notable limits on resolution of the image patterned in the resist layer. Reflection of radiation from the substrate/resist interface can produce variations in the radiation intensity in the resist during exposure, resulting in non-uniform photoresist linewidth upon development. Radiation also can scatter from the substrate/resist interface into regions of the resist where exposure is not intended, again resulting in linewidth variations. The amount of scattering and reflection will typically vary from region to region, resulting in further linewidth non-uniformity.
Reflection of activating radiation also contributes to what is known in the art as the "standing wave effect". To eliminate the effects of chromatic aberration in exposure equipment lenses, monochromatic or quasimonochromatic radiation is commonly used in resist projection techniques. Due to radiation reflection at the resist/substrate interface, however, constructive and destructive interference is particularly significant when monochromatic or quasi-monochromatic radiation is used for photoresist exposure. In such cases the reflected light interferes with the incident light to form standing waves within the resist. In the case of highly reflective substrate regions, the problem is exacerbated since large amplitude standing waves create thin layers of underexposed resist at the wave minima. The underexposed layers can prevent complete resist development causing edge acuity problems in the resist profile. The time required to expose the photoresist is generally an increasing function of resist thickness because of the increased total amount of radiation required to expose an increased amount of resist. However, because of the standing wave effect, the time of exposure also includes a harmonic component which varies between successive maximum and minimum values with the resist thickness. If the resist thickness is non-uniform, the problem becomes more severe, resulting in variable linewidth control.
Variations in substrate topography also give rise to resolution-limiting reflection problems. Any image on a substrate can cause impinging radiation to scatter or reflect in various uncontrolled directions, affecting the uniformity of resist development. As substrate topography becomes more complex with efforts to design more complex circuits, the effects of reflected radiation become more critical. For example, metal interconnects used on many microelectronic substrates are particularly problematic due to their topography and regions of high reflectivity.
Such radiation reflection problems have been addressed by the addition of certain dyes to photoresist compositions, the dyes absorbing radiation at or near the wavelength used to expose the photoresist. Exemplary dyes that have been so employed include the coumarin family, methyl orange and methanil yellow. Some workers have found that use of such dyes can limit resolution of the patterned resist image.
Another approach has been to use a radiation absorbing layer interposed between the substrate surface and the photoresist coating layer. See, for example, PCT Application WO 90/03598, and U.S. Pat. Nos. 4,910,122, 4,370,405 and 4,362,809, all of which are incorporated herein by reference for their teaching of antireflective (antihalation) compositions and use of the same. At least some prior antireflective coatings, however, suffer from poor adhesion to the overcoated photoresist layer and/or the underlying substrate surface. Such adhesion problems can severely compromise the resolution of the patterned photoresist image.
Thus, it would be desirable to have an antihalation coating composition that absorbs significantly incident and reflective exposure radiation, and that provides substantial standing wave attenuation. It would be further desirable to have an antihalation composition that can be coated on a microelectronic substrate and adhere well to both a subsequently applied photoresist coating layer and the underlying substrate surface.